Interpretive Summary: Yellow perch fillets are a high value product in the Great Lakes Regions. Availability shortfalls have been offset by increasing imports of lesser quality products. While domestic perch production is a growing industry, improvements in growth and feed efficiency are of considerable importance to the U.S. perch industry as such improvements will increase global competitiveness by decreasing imports. Against this background, current research is aimed at characterizing and understanding the hormones that regulate growth in this species. Very little is known regarding the involvement of hormones in the physiology of yellow perch. What is known is that the yellow perch exhibits a sexual size dimorphism (SSD) wherein females grow faster than males and studies have identified that the female sex steroid, estrogen, is a growth stimulator in yellow perch; however, the method by which estrogen promotes growth is not understood. To better understand how estrogen relates to growth, reproduction and sex in yellow perch, we examined levels of gene expression for the estrogen receptor and other hormones in yellow perch collected from Lake Erie by commercial trap nets. Overall, females had higher body weight, larger livers and higher levels of estrogen receptor-alpha (esr1), whereas males of the same age, but smaller, had higher levels of the growth regulating hormone Insulin-like growth factor-I(b) and estrogen receptor-beta (esr2a) in the liver. Season also influenced levels of growth-regulating hormones (pituitary growth hormone and liver IGF-Ib) with higher levels occurring in spring. For females, ovary size, liver esr1 and ovary cyp19a1a (an aromatase enzyme involved with estrogen production) mRNA levels were higher in autumn than the spring, and ovary cyp19a1a mRNA levels had a significant negative correlation with pituitary GH and liver IGF-Ib mRNA levels. Overall, the most significant (p=0.001) relationships were positive correlations between liver IGF-Ib and liver esr2a mRNA levels and IGF-Ib and liver cyp19a1a mRNA levels. Our findings demonstrate linkages between growth and reproductive development in this species and the involvement of an intricate relationship between estrogen and other growth-regulating hormones. Application of this knowledge in a selective breeding program should lead to improvements in yellow perch production efficiency.

Technical Abstract:
Yellow perch exhibit a sexual size dimorphism (SSD) where females grow faster and larger than males and estrogen preferentially stimulates growth in females. In an effort to gain more understanding of yellow perch endocrinology, real-time quantitative PCR (qPCR) was used to measure pituitary, liver and ovary sex-specific seasonal mRNA levels of several genes related to growth and/or sexual reproduction/development in adult yellow perch: growth hormone (GH), prolactin (PRL), somatolactin (SL), insulin-like growth factor (IGF-Ib), estrogen receptor-alpha (esr1), estrogen receptor-beta-a (esr2a) and ovarian aromatase (cyp19a1a). Adult fish were collected using trap nets from Lake Erie in the spring (May) and autumn (October) over two years and, along with the tissue mRNA levels, body weight, age, gonadasomatic index (GSI) and hepatasomatic index (HSI) were determined. Overall, females had higher body weight, HSI and liver esr1 mRNA level than males, while males had higher liver IGF-Ib, liver esr2a and liver cyp19a1a mRNA levels than females. Season had a significant effect on mRNA levels of growth-regulating hormones (pituitary GH and liver IGF-Ib) with higher levels occurring in spring, which also corresponded with higher liver cyp19a1a mRNA levels. For females, GSI, liver esr1 and ovary cyp19a1a mRNA levels were higher in autumn than the spring, and ovary cyp19a1a mRNA levels had a significant negative correlation with pituitary GH and liver IGF-Ib mRNA levels. Overall, the most significant (p<0.001) relationships were positive correlations between liver IGF-Ib and liver esr2a mRNA levels and IGF-Ib and liver cyp19a1a mRNA levels. This study shows that there are significant effects of season and sex on mRNA levels of key endocrine genes that are potentially involved in yellow perch SSD.